This project has as its major goals, the use of monoclonal technology to better understand the pathophysiology of immune cytopenias and to examine factors which regulate effector cell Fc receptor interaction with antibody displayed on cell surfaces. These studies will be approached by utilizing murine monoclonal antibodies to immunoglobulin G, its subclasses, IgM and complement to identify and quantitate the amount of these components on the surface of platelets and red cells from patients with immune cytopenia. These observations will be correlated with various clinical parameters including severity of disease and response to therapy. An indepth analysis of factors relevant to effector cell Fc receptor interaction with IgG displayed on target cell surfaces will be pursued by utilizing a number of model systems. First, platelet and red cell auto- and isoantibody sera will be utilized to sensitize target cells with a known display of IgG subclasses for study of Fc receptor binding and ADCC. Individual IgG subclass populations will be isolated from the above antisera using monoclonal antibody immunoaffinity columns to provide polyclonal, single subclass display on platelet and red cell surfaces for subsequent analysis of Fc receptor interaction. A third approach will be to utilize chimeric mouse/human monoclonal antibodies to tumor associated antigens. In this way, effector cell Fc receptor mediated ADCC to tumor cells can be characterized for each given IgG subclass. Finally, human monoclonal antibodies derived from hybridoma fusions of immune splenocytes will be used to characterize individual IgG subclass and antigen specificity factors as regards human monocyte Fc receptor interaction. The final thrust of the proposal will prepare multiple monoclonal antoantibodies from patients with immune thrombocytopenia. These antibodies will be utilized to isolate the platelet membrane antigens responsible for antibody binding in ITP. These same human monoclonal autoantibodies will be used to raise anti-idiotype reagents to allow characterization of cross- reactive idiotypes and idiotype diversity among patient platelet and red cell autoantibodies.